1. Field of the Invention
The present invention relates to an ink composition suitable for ink jet recording.
2. Background Art
Realization of images having good waterfastness and lightfastness and, at the same time, having no significant feathering or bleeding is important for printing of images using ink compositions.
In order to realize good waterfastness, as exemplified below, a combination of a cationic resin with an anionic dye has hitherto been used.
For example, Japanese Patent Laid-Open No. 119280/1987 discloses an ink comprising a hydroxyethylated polyethyleneimine polymer and a dye component. This publication describes that the combination of the hydroxyethylated polyethyleneimine polymer with the dye component can develop waterfastness. Japanese Patent Publication No. 91494/1995 discloses an ink comprising a hydroxyalkylated polyallylamine polymer and a dye. The claimed advantage of this ink is the development of waterfastness through the combination of the hydroxyalkylated polyallylamine polymer with the dye. In these two inks, howerver, since hydroxyalkyl groups having high hydrophilicity are contained in the polymer used, a further improvement is required to ensure satisfactory waterfastness of printed images.
Japanese Patent Laid-Open Nos. 255876/1990, 296878/1990, and 188174/1991 disclose ink compositions comprising a polyamine, with a molecular weight of not less than 300, having primary amino groups, an anionic dye, and a stability-imparting agent. These publications describe that the combination of the primary amino group with the anionic dye can develop waterfastness. Disadvantageously, however, the resins used in these publications strongly attack the dye. Specifically, according to studies conducted by the present inventors, for example, when this ink is allowed to stand at high temperatures, in some cases, the dye is decomposed or otherwise the photodecomposition of the print is accelerated to render the lightfastness lower than that of an ink containing only the dye. This tendency is significant in ink compositions containing some magenta dyes. In the case of ink compositions other than the magenta dye-containing ink compositions, the storage stability of ink compositions and the lightfastness and waterfastness of prints could be balanced by bringing the content of the primary amino group to not more than about 20% by mole based on the total content of the side chain group in the polymer. However, in the case of the ink compositions containing the specific magenta dye, this level of amount of the resin added is insufficient and cannot offer satisfactory waterfastness of printed images.
Japanese Patent Laid-Open No. 305011/1995 discloses a water-base ink comprising a basic water-soluble polymer, an anionic dye with the counter ion being a volatile base, and a buffering agent with the counter ion being a volatile base. This publication describes that, in the ink, the volatile base prevents the dissociation of the polymer and, after printing, the volatile base on paper is evaporated to allow a salt-forming reaction between the polymer and the dye to proceed, thereby developing waterfastness.
Japanese Patent Laid-Open No. 238783/1987 discloses an ink jet recording sheet comprising a homopolymer of an acid salt of diallylamine or an acid salt of monoallylamine or a copolymer of the acid salt of diallylamine with the acid salt of monoallylamine. The claimed advantage of this ink jet recording sheet is that an insolubilized reaction of the polymer with the dye takes place on the recording medium to develop waterfastness. In the recording method using the recording medium containing the cationic resin disclosed in this publication, since the ink per se does not have waterfastness, use of a recording medium other than described in the publication cannot develop the waterfastness in the printed image.
Further, the formation of images having no significant feathering or bleeding, especially the prevention of color-to-color bleeding involved in use of two or more color inks, is also important.
The present inventors have now found that an ink composition containing a cationic water-soluble resin having a specific structure can realize images having excellent waterfastness and lightfastness and, at the same time, having no significant feathering or bleeding. The present invention has been made based on such finding.
Accordingly, it is an object of the present invention to provide an ink composition which realize images having excellent waterfastness and lightfastness and, at the same time, having no significant feathering or bleeding, especially no significant color-to-color bleeding.
In particular, it is an object of the present invention to provide an ink composition which can be advantageously used for ink jet recording.
According to one aspect of the present invention, there is provided an ink composition comprising at least an alkali-soluble colorant, a water-soluble organic solvent, water, and a cationic water-soluble resin,
said cationic water-soluble resin comprising repeating units (a) represented by formula (a) in formula (I) and repeating units (b1) represented by formula (b1) in formula (I) or repeating units (b2) represented by formula (b2) in formula (I): 
wherein
R1 and R2, which may be the same or different, each represent a C1-5 alkyl group;
R3 represents a hydrogen atom or a C1-5 alkyl group; and
m and n are each 0, 1, or 2.
Ink Composition
The ink composition according to the present invention may be used in recording methods using an ink composition. Recording methods using an ink composition include, for example, an ink jet recording method, a recording method using writing utensils, such as pens, and other various recording methods. Particularly preferably, the ink composition according to the present invention is used in the ink jet recording method.
The ink composition according to the present invention basically comprises at least an alkali-soluble colorant, a water-soluble organic solvent, water, and a cationic water-soluble resin. In this case, the cationic water-soluble resin comprises repeating units (a) represented by formula (a) in formula (I) and repeating units (b1) represented by formula (b1) in formula (I) or repeating units (b2) represented by formula (b2) in formula (I).
The resin comprising units (a) and units (b1) will be hereinafter referred to as the xe2x80x9cresin of the first embodiment,xe2x80x9d and the resin comprising units (a) and units (b2) will be hereinafter referred to as the xe2x80x9cresin of the second embodiment.xe2x80x9d
Resin of First Embodiment
The ink composition comprising the resin of the first embodiment according to the present invention can realize images having no significant feathering or bleeding and, at the same time, having excellent waterfastness. Although the reason why this effect can be attained has not been fully elucidated yet, it is believed as follows. The cationic water-soluble resin of the first embodiment contained in the ink composition according to the present invention is stably in the state of dissolution in the ink composition. Upon the deposition of this ink onto a recording medium, the cationic water-soluble resin causes electrostatic interaction with the colorant. This resin also interacts with the recording medium and thus can be stably fixed onto the recording medium. The fixation of the resin onto the recording medium is considered to permit the colorant, together with the resin, to be fixed onto the recording medium, thereby imparting the waterfastness to printed images. In particular, even though the pH value of the recording medium is on alkaline side, good waterfastness can be realized. Further, it is considered that good fixation of the colorant onto the recording medium can realize images having no significant feathering or bleeding.
Further, the ink composition according to the present invention can realize images having excellent lightfastness. Although the reason why the ink composition can provide lightfast images has not been fully elucidated yet, it is considered that the copresence of repeating units having primary amino groups and repeating units having tertiary amino groups in an identical molecule can markedly reduce the acceleration of photodecomposition of prints caused by the primary amino groups. Specifically, the primary amino group has stronger electrostatic interaction with the colorant than the tertiary amino group. Therefore, use of the resin having only primary amino groups often accelerates photodecomposition of prints. That is, images yielded by an ink composition containing a resin having only the primary amino groups have poor lightfastness. On the other hand, the copresence of the primary amino group and the tertiary amino group in an identical molecule inhibits the electrostatic interaction of the primary amino group and can alleviate the acceleration of the photodecomposition. This is also estimated, for example, from the fact that, when an ink is prepared using a mixture of a resin having only primary amino groups with a resin having tertiary amino groups, photodecomposition of prints, obtained using this ink, caused by the primary amino group is not very alleviated.
Furthermore, the ink composition of the present invention can realize images having excellent waterfastness. An ink composition containing a resin having primary amino groups and an ink composition containing a resin having tertiary amino groups each have good storage stability and, in addition, can realize images having good waterfastness on acidic paper. Images yielded on neutral or alkaline paper by the ink composition containing the resin having tertiary amino groups have slightly poor waterfastness, while the ink composition containing the resin having primary amino groups can realize images having good waterfastness even on neutral or alkaline paper. The resin having both primary amino groups and tertiary amino groups in the same molecule as used in the present invention can provide an advantage that good waterfastness can be realized on acidic paper, as well as on neutral or alkaline paper. The present inventors have experimentally confirmed that this advantage cannot be offered by an ink composition containing a mere mixture of the resin having only primary amino groups with the resin having only tertiary amino groups.
In unit (a), R1 and R2 may be the same or different and each independently represent a C1-5 alkyl group, preferably a C1-5 alkyl group, more preferably a methyl group. Most preferably, R1 and R2 simultaneously represent a methyl group.
m and n are each 0, 1, or 2. Preferably, m and n are simultaneously 1.
According to a preferred embodiment of the present invention, the unit (a) to unit (b1) ratio is in the range of 1:4 to 4:1. Use of this cationic water-soluble resin can markedly alleviate the acceleration of photodecomposition of prints caused by the primary amino group and can retain good waterfastness.
Resin of Second Embodiment
The ink composition containing the resin of the second embodiment according to the present invention can realize images having no significant feathering or bleeding and, at the same time, having excellent waterfastness. Although the reason for this has not been fully elucidated yet, it is believed as follows. The cationic water-soluble resin of the second embodiment contained in the ink composition according to the present invention is stably in the state of dissolution in the ink composition. Upon the deposition of this ink onto a recording medium, the cationic water-soluble resin causes electrostatic interaction with the colorant. This resin also interacts with the recording medium and thus can be stably fixed onto the recording medium. The fixation of the resin onto the recording medium is considered to permit the colorant, together with the resin, to be fixed onto the recording medium, thereby imparting the waterfastness to printed images on various recording media. In particular, even when the pH of the recording medium is on neutral or alkaline side, good waterfastness can be realized. Further, it is considered that good fixation of the colorant onto the recording medium can realize images having no significant feathering or bleeding.
Resins comprising only units (b2) can realize good waterfastness on acidic paper, as well as on neutral or alkaline paper, but on the other hand, the lightfastness of printed images and the storage stability of the ink are somewhat poor. On the other hand, an ink composition containing a resin having tertiary amino groups can yield highly lightfast images and has excellent storage stability. As described above, however, images yielded on neutral or alkaline paper by this ink composition are somewhat poor in waterfastness. An ink composition containing a resin having tertiary amino groups and units (b2) in the same molecule can realize images having good waterfastness and lightfastness and, in addition, has excellent storage stability. The reason for this has not been fully elucidated yet. The present inventors, however, have experimentally confirmed the fact that this advantageous effect cannot be attained by an ink composition containing a mere mixture of a resin comprising only units (a) with a resin comprising only units (b2).
The ink composition according to the present invention can simultaneously realize good waterfastness on various recording media and good storage stability. Specifically, regarding the storage stability, any deposit is not created during storage for a long period of time. Although the reason for this has not been fully elucidated yet, this effect is considered attributable to the fact that the cationic water-soluble resin contains units (a) and units (b2) in the same molecule. The present inventors have experimentally confirmed that, unlike the ink composition of the present invention, an ink containing a mere mixture of a resin having only unsubstituted diallylamino groups with a resin having only dialkylallylamino groups cannot realize good waterfastness and storage stability.
In the cationic water-soluble resin used in the present invention, R1, and R2 may be the same or different and represent a C1-5 alkyl group, preferably a C1-3 alkyl group, more preferably a methyl group. R3 represents a hydrogen atom or a C1-5 alkyl group, preferably a C1-3 alkyl group, more preferably a methyl group. Most preferably, R1, R2, and R3 simultaneously represent a methyl group.
n is 0, 1, or 2, preferably 1.
The unit (a) to unit (b2) ratio in the resin is preferably 2:8 to 7:3. Use of this cationic water-soluble resin can offer higher level of ink storage stability and waterfastness on neutral or alkaline paper. These two types of repeating units (a) and (b2) may be present in a block or random form in the resin.
According to a preferred embodiment of the present invention, the weight average molecular weight of the cationic water-soluble resin is preferably about 250 to 20,000, more preferably about 400 to 10,000, most preferably 500 to 3,000.
According to a preferred embodiment of the present invention, the cationic water-soluble resin contained in the ink composition may contain structural units other than units (a) and units (b1) or units (b2). The presence of the third unit can improve various properties of the cationic water-soluble resin. Monomers for providing the third unit usable herein include propylene, isobutylene, styrene, vinyl chloride, vinylidene chloride, vinyl acetate, acrylic acid or acrylic esters (for example, lower alkyl esters), methacrylic acid or methacrylic esters (for example, lower alkyl esters), acrylonitrile, methyl vinyl ether, vinylpyrrolidone, or sulfur dioxide. The content of units derived from these monomers in the cationic water-soluble resin is not particularly limited. However, the ratio of the number of moles of the additional units to the total number of moles of the units (a) and (b1) or (b2) is preferably not more than 0.7, more preferably not more than 0.3. These additional units may be present in a block or random form in the cationic water-soluble resin.
According to a preferred embodiment of the present invention, the ink composition contains a water-soluble resin other than the cationic water-soluble resin. The additional water-soluble resin is preferably a nonionic water-soluble resin, and examples thereof include polyacrylamide, hydroxy esters of polymethacrylic acid, such as polyhydroxyethyl methacrylate, polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene glycol. The addition of these water-soluble resin can further stabilize the ink composition.
Colorant and Solvent
The alkali-soluble colorant contained in the ink composition according to the present invention may be either a dye or a pigment. The term xe2x80x9calkali-solublexe2x80x9d used herein means that the colorant can be dissolved in an alkaline medium. The water-soluble group contained in the molecule may be an acidic or basic dissociative group or nondissociative functional group, or alternatively a plurality of kinds of these groups are present in the molecule. Alkali-soluble colorants may be soluble in acidic solutions so far as they are soluble in alkalis.
The content of the colorant may be properly determined. For example, the content of the colorant is preferably 0.5 to 20% by weight based on the total weight of the ink composition. This is because a colorant content falling within the above range can provide printed images having satisfactory optical density and permits the viscosity of the ink to be easily adjusted to a value suitable for ink jet recording.
More preferably, the colorant is selected from organic dyes or organic pigments. Use of organic dyes or organic pigments can offer high color density per weight and vivid colors.
Dyes are organic color materials soluble in water, and preferred dyes are those that fall into categories of acidic dyes, direct dyes, reactive dyes, soluble vat dyes, and food dyes according to the color index. Further, colorants, insoluble in neutral water, falling within categories of oil-soluble dyes and basic dyes according to the color index may also be used so far as they are soluble in aqueous alkali solutions.
On the other hand, pigments may be generally selected from those falling within categories of pigments according to the color index. Although pigments are generally regarded as water-insoluble organic color materials, some pigments are soluble in alkalis and may be used in the present invention.
Specific examples of dyes and pigments usable herein include:
yellow dyes and pigments, such as C.I. Acid Yellow 1, 3, 11, 17, 19, 23, 25, 29, 36, 38, 40, 42, 44, 49, 59, 61, 70, 72, 75, 76, 78, 79, 98, 99, 110, 111, 127, 131, 135, 142, 162, 164, and 165, C.I. Direct Yellow 1, 8, 11, 12, 24, 26, 27, 33, 39, 44, 50, 55, 58, 85, 86, 87, 88, 89, 98, 110, 132, 142, and 144, C.I. Reactive Yellow 1, 2, 3, 4, 6, 7, 11, 12, 13, 14, 15, 16, 17, 18, 22, 23, 24, 25, 26, 27, 37, and 42, C.I. Food Yellow 3 and 4, C.I. Solvent Yellow 15, 19, 21, 30, and 109, and C.I. Pigment Yellow 23;
red dyes and pigments, such as C.I. Acid Red 1, 6, 8, 9, 13, 14, 18, 26, 27, 32, 35, 37, 42, 51, 52, 57, 75, 77, 80, 82, 85, 87, 88, 89, 92, 94, 97, 106, 111, 114, 115, 117, 118, 119, 129, 130, 131, 133, 134, 138, 143, 145, 154, 155, 158, 168, 180, 183, 184, 186, 194, 198, 209, 211, 215, 219, 249, 252, 254, 262, 265, 274, 282, 289, 303, 317, 320, 321, and 322, C.I. Direct Red 1, 2, 4, 9, 11, 13, 17, 20, 23, 24, 28, 31, 33, 37, 39, 44, 46, 62, 63, 75, 79, 80, 81, 83, 84, 89, 95, 99, 113, 197, 201, 218, 220, 224, 225, 226, 227, 228, 229, 230, and 231, C.I. Reactive Red 1, 2, 3, 4, 5, 6, 7, 8, 11, 12, 13, 15, 16, 17, 19, 20, 21, 22, 23, 24, 28, 29, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 45, 46, 49, 50, 58, 59, 63, and 64, C.I. Solubilized Red 1, C.I. Food Red 7, 9, and 14, and C.I. Pigment Red 41, 48, 54, 57, 58, 63, 68, and 81;
blue dyes and pigments, such as C.I. Acid Blue 1, 7, 9, 15, 22, 23, 25, 27, 29, 40, 41, 43, 45, 54, 59, 60, 62, 72, 74, 78, 80, 82, 83, 90, 92, 93, 100, 102, 103, 104, 112, 113, 117, 120, 126, 127, 129, 130, 131, 138, 140, 142, 143, 151, 154, 158, 161, 166, 167, 168, 170, 171, 182, 183, 184, 187, 192, 199, 203, 204, 205, 229, 234, 236, and 249, C.I. Direct Blue 1, 2, 6, 15, 22, 25, 41, 71, 76, 77, 78, 80, 86, 87, 90, 98, 106, 108, 120, 123, 158, 160, 163, 165, 168, 192, 193, 194, 195, 196, 199, 200, 201, 202, 203, 207, 225, 226, 236, 237, 246, 248, and 249, C.I. Reactive Blue 1, 2, 3, 4, 5, 7, 8, 9, 13, 14, 15, 17, 18, 19, 20, 21, 25, 26, 27, 28, 29, 31, 32, 33, 34, 37, 38, 39, 40, 41, 43, 44, and 46, C.I. Solubilized Vat Blue 1, 5, and 41, C.I. Vat Blue 29, C.I. Food Blue 1 and 2, C.I. Basic Blue 9, 25, 28, 29, and 44, and C.I. Pigment Blue 1 and 17; and
black dyes and pigments, such as C.I. Acid Black 1, 2, 7, 24, 26, 29, 31, 48, 50, 51, 52, 58, 60, 62, 63, 64, 67, 72, 76, 77, 94, 107, 108, 109, 110, 112, 115, 118, 119, 121, 122, 131, 132, 139, 140, 155, 156, 157, 158, 159, and 191, C.I. Direct Black 17, 19, 22, 32, 35, 38, 51, 56, 62, 71, 74, 75, 77, 94, 105, 106, 107, 108, 112, 113, 117, 118, 132, 133, 146, 154, 168, 171, and 195, C.I. Reactive Black 1, 3, 4, 5, 6, 8, 9, 10, 12, 13, 14, and 18, C.I. Solubilized Vat Black 1, and C.I. Food Black 2. These colorants may be used alone or as a mixture of two or more.
According to the ink composition of the present invention, water is a main solvent. Water may be pure water obtained by ion exchange, ultrafiltration, reverse osmosis, distillation or the like, or ultrapure water. Further, water, which has been sterilized by ultraviolet irradiation or by addition of hydrogen peroxide, is suitable because, when the ink composition is stored for a long period of time, it can prevent the growth of mold or bacteria.
The ink composition according to the present invention may contain a basic material. Examples of basic materials usable herein include: inorganic bases, such as sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, barium hydroxide, strontium hydroxide, radium hydroxide, berylium hydroxide, magnesium hydroxide, and ammonia; mono-, di- or tri-lower alkylamines, such as ethylamine, diethylamine, triethylamine, propylamine, dipropylamine, diisopropylamine, tert-butylamine, dibutylamine, diisobutylamine, isopropylamine, sec-butylamine, and pentylamine; lower alkyl lower hydroxyalkoxyamines, such as 3-ethoxypropylamine and 3-methoxypropylamine; lower alkyl lower alkoxyamines, such as 3-ethoxypropylamine and 3-methoxypropylamine; mono-, di- or tri-lower hydroxyalkylamines, such as 2-aminoethanol, 2-(dimethylamino)ethanol, 2-(diethylamino)ethanol, diethanolamine, N-butyldiethanolamine, triethanolamine, aminomethylpropanol, and triisopropanolamine; and organic amines, such as iminobispropylamine, 3-diethylaminopropylamine, dibutylaminopropylamine, methylaminopropylamine, dimethylaminopropanediamine, and methyliminobispropylamine. These basic materials can function so that the cationic water-soluble resin and the colorant can be stably dissolved and held in the ink composition. For example, when an ink is prepared using the cationic water-soluble resin according to the present invention in combination with a specific colorant, mere mixing is often insufficient for dissolution of these ingredients in the ink. The addition of the basic material permit the resin and the colorant to be stably dissolved in the ink.
In the ink composition according to the present invention, the term xe2x80x9cwater-soluble organic solventxe2x80x9d refers to an aqueous medium which can dissolve solutes of the ink, such as a colorant. Preferably, the water-soluble organic solvent is selected from water-soluble solvents having a lower vapor pressure than water. Example thereof include: polyhydric alcohols, such as ethylene glycol, propylene glycol, butanediol, pentanediol, 2-butene-1,4-diol, 2-methyl-2,4-pentanediol, glycerin, 1,2,6-hexanetriol, diethylene glycol, and dipropylene glycol; ketones, such as acetonylacetone; xcex3-butyrolactone; esters, such as diacetin and triethyl phosphate; lower alkoxy lower alcohols, such as 2-methoxyethanol and 2-ethoxyethanol; and furfuryl alcohol, tetrahydrofurfuryl alcohol, and thiodiglycol. Water-soluble organic solvents usable herein include those which are liquid at room temperature, those which are solid at room temperature and, upon heat melting, can function as solvents, and those which, when used in combination with an aqueous solution or other solvent, can function as solvents. Since the vapor pressure of the organic solvent is lower than that of pure water, even though drying of the ink proceeds at the front end of an ink jet head, the proportion of the organic solvent in the ink is not lowered. As a result, the dissolving power of the organic solvent is not lowered, and the ink can be stably kept.
The amount of the water-soluble organic solvent may be properly determined. For example, the amount of the water-soluble organic solvent added is preferably 5 to 50% by weight based on the total amount of the ink.
According to a preferred embodiment of the present invention, the ink composition may further contain the following organic solvent. Specific examples of additional organic solvents usable herein include imidazole, methylimidazole, hydroxyimidazole, triazole, nicotinic amide, dimethylaminopyridine, xcex5-caprolactam, 1,3-dimethyl-2-imidazolidinone, lactic amide, sulfolane, dimethylsulfoxide, 1,3-propanesultone, methyl carbamate, ethyl carbamate, 1-methylol-5,5-dimethylhydantoin, hydroxyethylpiperazine, piperazine, ethyleneurea, propyleneurea, ethylene carbonate, propylene carbonate, N-methyl-2-pyrrolidinone, 2-pyrrolidinone, acetamide, formamide, dimethylformamide, N-methylformamide, dimethylacetamide, guanidine, guanidinoacetic acid, guanitidine, aminoguanidine, canavanine, argininosuccinic acid, arginine, biguanide, pyrazine, hexahydropyrazine, triazine, pyridazine, cytosine, 1,2,3-triazole, 1,2,4-triazole, pyrazole, tetrazole, thiazole, 1,2,3-thiadiazole, purine, guanine, guanosine, methylguanine, caffeine, and xanthine. The addition of these organic solvents can prevent, for example, precipitation even upon cooling of the ink and enables printing to be stably performed even under such environment.
If necessary, the ink composition according to the present invention may further comprise assistants commonly used in ink for ink jet recording. Examples of assistants usable herein include penetrants, viscosity modifiers, surface tension modifiers, hydrotropy agents, humectants, pH adjustors, antimolds, chelating agents, preservatives, and rust preventives. When the ink is used in an ink jet recording method utilizing electrification of the ink, a specific resistance modifier may be added which is selected from inorganic salts, such as lithium chloride, sodium chloride, and ammonium chloride.
Penetrants usable herein include: lower alcohols, such as ethanol, isopropanol, butanol, and pentanol; cellosolves, such as ethylene glycol monobutyl ether; carbitols, such as diethylene glycol monobutyl ether and triethylene glycol monobutyl ether; and surfactants. Surface tension modifiers usable herein include: alcohols, such as diethanolamine, triethanolamine, glycerin, and diethylene glycol; and nonionic, cationic, anionic, or amphoteric surfactants.
Preferred hydrotropy agents usable herein include urea, alkylureas, ethyleneurea, propyleneurea, thiourea, guanidine acid salts, and tetraalkylammonium halides.
Regarding the humectant, glycerin, diethylene glycol or the like may be added as an agent which serves also as the water-soluble organic solvent. Furthers, saccharides, such as maltitol, sorbitol, gluconic lactone, maltose and the like may be added.
Regarding the pH adjustor, the basic material described above may be added as a pH adjustor which serves also as the basic material.
Ink Set
As described above, the ink composition according to the present invention contains a cationic water-soluble resin. On the other hand, according to another aspect of the present invention, there is provided an ink set comprising at least a black ink, a yellow ink, a cyan ink, and a magenta ink. These inks are ink compositions according to the present invention. In this case, the black ink contains as the colorant a member selected from the group consisting of C.I. Direct Black 19, 35, 154, 168, 171, and 195 and C.I. Food Black 2; the yellow ink contains as the colorant a member selected from the group consisting of C.I. Direct Yellow 50, 55, 86, 132, 142, and 144 and C.I. Acid Yellow 23; the cyan ink contains as the colorant a member selected from the group consisting of C.I. Direct Blue 86, 87, and 199 and C.I. Acid Blue 9 and 249; and the magenta ink contains as the colorant a member selected from the group consisting of C.I. Direct Red 9 and 227, C.I. Acid Red 52 and 289, and dyes represented by formula (II): 
wherein
R4 and R5 each represent a hydrogen atom, a C1-5 alkyl group, an aryl group, a C1-5 alkoxy group, a phenoxy group or a derivative thereof, a triazine ring or a derivative thereof, a carboxyl group or a salt thereof, or a sulfonyl group or a derivative thereof;
X represents a hydrogen atom or a halogen atom; and
Y1 and Y2, which may be the same or different, each represent an alkali metal atom, ammonium, or a C1-5 alkylammonium.
Use of this ink set can realize the formation of full-color images having excellent waterfastness on various recording media. Although the reason for this has not been fully elucidated yet, the excellent waterfastness is believed to derive from strong electrostatic interaction between the above group of dyes and the cationic water-soluble resin.
Among a group of dyes represented by formula (II), a dye represented by formula (III) is preferred. This dye falls into the category of C.I. Acid Red 249 according to the color index. 
Further, among the dyes represented by formula (II), a dye represented by formula (IV) is also preferred. 
Use of the ink composition according to the present invention in combination with the ink composition containing an anionic material can realize images having no significant color-to-color bleeding between two inks. It is considered that, upon contact of the two ink compositions on a recording medium, the cationic water-soluble resin in the ink composition according to the present invention is reacted with the anionic material to create a precipitate which can inhibit further spreading of the ink composition on the recording medium, resulting in the realization of printed images having no significant color-to-color bleeding.
Therefore, according to a further aspect of the present invention, there is provided an ink set comprising: the ink composition according to the present invention as a first ink composition; and an anionic material-containing ink composition as a second ink composition.
The ink set generally comprises a yellow ink, a magenta ink, a cyan ink, and a black ink. According to a preferred embodiment of the present invention, the ink compositions according to the present invention are used as the yellow ink, the magenta ink, and the cyan ink with an ink composition containing an anionic material being used as the black ink, or alternatively, the ink composition according to the present invention is used as the black ink with anionic material-containing ink compositions being used as the yellow ink, the magenta ink, and the cyan ink.
In this case, the second ink composition, that is, the anionic material-containing composition basically comprises a colorant, a water-soluble organic solvent, water, an anionic material, and a basic material.
According to a preferred embodiment of the present invention, the anionic material may be an anionic resin. Examples of preferred anionic resins include those which have anionic functional groups, such as sulfonic acid, carboxyl, phosphoric acid, or hydroxyl groups, and, together with the basic material, form a water-soluble salt. Specific examples thereof include: cellulose derivatives, such as carboxymethylcellulose and viscose; naturally occurring polymeric materials, such as alginic acid, gum arabic, tragacanth, and lignin sulfonic acid; starch derivatives, such as phosphorylated starch and carboxymethyl starch salt; and synthetic polymers, such as polyacrylic acid, polymethacrylic acid, polyvinylsulfuric acid, polyvinylsulfonic acid, condensed naphthalenesulfonic acid, ethylene/acrylic acid copolymer, styrene/acrylic acid copolymer, styrene/methacrylic acid copolymer, acrylic ester/acrylic acid copolymer, acrylic ester/methacrylic acid copolymer, methacrylic ester/acrylic acid copolymer, methacrylic ester/methacrylic acid copolymer, styrene/itaconic acid copolymer, itaconic ester/itaconic acid copolymer, vinylnaphthalene/acrylic acid copolymer, vinylnaphthalene/methacrylic acid copolymer, vinylnaphthalene/itaconic acid copolymer, phenolic resin, and copolymers thereof.
According to the present invention, the colorant used in the second ink composition may be a pigment dispersed in the anionic resin.
According to another preferred embodiment of the present invention, the anionic material is a pigment having on its surface an anionic functional group. The formation of a salt of the anionic group with the basic material enables pigment particles to be dispersed in water. Anionic functional groups usable herein include, for example, sulfonic acid, carboxyl, and phosphoric acid groups. The pigment having on its surface an anionic functional group may be obtained by grafting the functional group onto the surface of the pigment. Further, the pigment may be a commercially available one, and preferred examples thereof include Microjet CW-1 and Microjet CW-2 (tradename; manufactured by Orient Chemical Industries, Ltd.).
Examples of basic materials usable herein include those recited above as usable in the ink composition of the present invention.
In the anionic material-containing ink composition constituting the ink set according to the present invention, the other ingredients may be fundamentally the same as those constituting the ink composition of the present invention, except that the cationic water-soluble resin is omitted therefrom.